1. Field of the Invention
This invention relates to a transparent dome for electro-optic sensors such as found on airborne platforms, such as a missile or airplane. More particularly, the present invention relates to a one piece extended dome having a spanning angle greater than 180 degrees that is integrally formed of a Nano/Nano class of Nanocomposite Optical Ceramic (NNOC) material.
2. Description of the Related Art
Airborne platforms that carry electro-optical (EO) sensors for such tasks as target acquisition, identification, guidance, etc are generally provided with a transparent dome to protect the optical system. Guided projectiles, such as missiles, rockets and shells, are generally provided with a transparent dome at their front. Behind this dome, and within the body of the projectile, an EO seeker is provided for capturing electro-magnetic radiation (EMR) from the target, and conveying target information (e.g. bearing or images) to a guidance system, which in turn guides the projectile to an object or point within the captured images. Aircraft such as planes or helicopters may be provided with a directed infrared countermeasures (DIRCM) system to jam a missile seeker. This system may be mounted on the belly, tail section or elsewhere on the aircraft behind a protective transparent dome.
The dome is generally made of a transparent material that can sustain the aerodynamic and thermal stresses that may be applied on it during the missile or aircraft flight. In many conventional applications the dome is made of Sapphire. Other materials such as silica, aluminum oxynitride (ALON) and nanocomposites have found limited application. US Patent Pub. 2009/0283720 discloses the use of a nanocomposite optical ceramic material to form the window for an ogive-shaped nose cone. As shown in FIG. 2 of 2009/0283720, the nanocomposite material comprises particles of a nano-dispersoid incorporated into the grains of a host matrix material of the type listed in Table 1. As shown the fused polycrystalline grains of the matrix material are not nano-sized. The incorporation of the nano-dispersoid particles into the matrix serves to strengthen the host matrix material. The host matrix material determines the dome's optical properties. The nano-dispersoids are kept small to avoid scattering the IR light and affecting the optical properties.
The size of the field of regard (FOR) that can be obtained by the EO seeker depends on the spanning angle of the dome used. The term “spanning angle” when used herein refers to the actual angular portion that the dome spans without vignetting with respect to a full sphere whose spanning angle is 360°. The angle measured from the longitudinal axis through the center of the dome to the edge of the FOR is one-half the spanning angle and is referred to as the “look angle.” Conventional missile domes are made of at most approximately half a sphere size. Therefore, when a conventional optical seeker is provided at the center of dome, and if it is mounted on one, two, or more axes gimbals, this optical sensing unit of the prior art can theoretically view a field of regard of at most 180 degrees. Although it is known that the size of the field of regard depends on the spanning angle of the dome, domes spanning more than half a sphere (180°) are generally not in use. This is so, mainly due to technological obstacles in producing Sapphire and other materials domes with large spanning angles and with the required strength, optical and thermal characteristics. More particularly, production of a Sapphire dome having a spanning angle substantially larger than 180° if at all possible, is a very expensive and complicated task.
As said, the maximal active field of operation of a guided projectile is limited to within the field of regard. In order to increase the field of operation of a guided projectile, it is therefore necessary to increase its field of regard, which in turn depends on the spanning angle of the dome. Manufacturing techniques have been developed to produce domes in which the FOR is greater than 180 degrees. These techniques separately fabricate two pieces, typically a spherical portion similar to a conventional dome and an extended portion, and attach the two pieces. The attachment process creates an optical interface along the line of attachment, which has the deleterious effect of producing a discontinuity as the EO seeker scans the FOR. Such a discontinuity poses a risk the seeker may lose track on the target. As a consequence, such extended domes are generally not in use.
U.S. Pat. No. 4,291,848 entitled “Missile Seeker Optical System” discloses a sphero-conical dome 12 of silica glass that provides off-boresight viewing angles up to 135 degrees. A conical portion 26 is attached to a spherical portion 28 to extend the field of regard. Both inner and outer cone surfaces are tangent to the spherical surfaces of the portion 28 at the point of attachment (col 2, lines 28-30). Corrector lenses are positioned so that the combined conical dome and corrector lens have the same optical power as the spherical portion of the dome, so focus is maintained.
U.S. Pat. No. 7,335,865 entitled “Dome” discloses a spherical dome having a spanning angle larger than 180 degrees. The entire extended dome is spherical obviating the need for corrector lens. The dome is manufactured by growing from single crystals of a ceramic material a first dome portion, which is a portion of a sphere, and a second dome portion, which is a complementary sphere-portion for the first dome portion. The complementary dome portion is attached to the first dome portion thereby forming a front dome having a spanning angle larger than 180 degrees.